1. Field of the Invention
This invention relates to polyimides and polyamic acid precursors thereof which may be thermally crosslinked and which exhibit improved thermooxidative stability and processing properties.
2. Description of Related Art
Polyimides are widely used in the aero-space industry and in the electronics industry because of their toughness, low density, thermal stability, radiation resistance and mechanical strength. Polyimides based on the polymer condensation product of aromatic diamines and dianhydrides containing a hexafluoroisopropylidene linking group are disclosed, for example, in U.S. Pat. Nos. 3,356,648, 3,959,350 and 4,592,925. These polyimides may be produced by first preparing a polyamic acid by reacting the diamine and dianhydride in a suitable solvent to produce the polyamic acid, followed by cyclization of the polyamic acid to the polyimide.
U.S. Pat. No. 3,763,211 teaches the preparation of bis(aminophenoxy) benzonitrile monomers such as 2,6-di(4-aminophenoxy) benzonitrile and suggests the use thereof in the preparation of polyamide or polyimide polymers by reaction with an aromatic acid chloride or an aromatic dianhydride respectively.
Polyimides such as prepared above are either soluble or swellable in many common organic solvents such as toluene, n-methyl-pyrrolidone, acetone, methyl ethyl ketone, diglyme, gamma-butyrolactone, propylene glycol methyl ether and like materials. While this property of solubility can be advantageous in certain applications such as solution spinning, spray coating and the preparation of polymer films by casting, it can also be a detriment in certain applications where the surface of the polyimide must be cleaned or otherwise treated with organic solvents after application to a substrate, or where composites and shaped articles come into contact with solvents or fuels. For example, polyimides are commonly used in flexible printed circuitry applications wherein the film is laminated to a backing such as copper, then coated with a photoresist, exposed and etched. Often times the polyimide surface must first be cleaned with organic solvents to remove dust and other impurities prior to the application of the resists, such as disclosed in U.S. Pat. No. 3,833,436. Such a treatment can result in a cracking or partial dissolution of the soluble or swellable polyimide which will detract from the effectiveness of the polyimide as an insulating or dielectric layer. In addition, composite articles or shaped parts used in the aircraft industry (fuel tank liners, for example) must be inert to solvents used in their cleaning and to fuels with which they may come in contact.
Accordingly, it is an object of this invention to provide a method for rendering coatings, films, composites and other articles based on solvent soluble or swellable polyimides insoluble after application to a surface or after shaping.
It is also an object of this invention to prepare cross-linkable polyimides derived from aromatic dianhydrides containing the hexafluoroisopropylidene linking group.